Nickel-containing steel for low temperature

ABSTRACT

A nickel-containing steel for low temperature according to an aspect of the present invention has a chemical composition within a predetermined range, in which a metallographic structure of a thickness middle portion contains 2.0 vol % to 20.0 vol % of an austenite phase, an average grain size of prior austenite grains is 3.0 μm to 15.0 μm, an average aspect ratio of the prior austenite grains is 1.0 to 2.4, a plate thickness is 4.5 mm to 30 mm, the chemical composition and the average grain size of the prior austenite grains are further limited depending on the plate thickness, a yield stress at room temperature is 460 MPa to 710 MPa, and a tensile strength at the room temperature is 560 MPa to 810 MPa.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No. 16/758,010filed on Apr. 21, 2020, which is the National Phase of PCT InternationalApplication No. PCT/JP2017/038615, filed on Oct. 26, 2017, all of whichare hereby expressly incorporated by reference into the presentapplication.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a nickel-containing steel for lowtemperature, that is, a steel containing nickel (Ni) suitable for lowtemperature at around −253° C.

RELATED ART

In recent years, expectations for the use of liquid hydrogen as cleanenergy have increased. A steel plate used for a tank that stores andtransports a liquefied gas such as liquid hydrogen requires excellentlow temperature toughness, and austenitic stainless steel which is lesslikely to undergo brittle fracture has been used. Although austeniticstainless steel has sufficient low temperature toughness, the yieldstress of a general-purpose material at room temperature is about 200MPa.

In a case where austenitic stainless steel with insufficient strength isapplied to a liquid hydrogen tank, there is a limit to the increase inthe size of the tank. Furthermore, when the yield stress of the steel isabout 200 MPa, the plate thickness thereof needs to exceed 30 mm whenthe tank is increased in size. Therefore, an increase in the weight ofthe tank and an increase in manufacturing cost are problems. For suchproblems, for example, an austenitic high Mn stainless steel having aplate thickness of 5 mm and a 0.2% proof stress of 450 MPa or more atroom temperature is proposed (for example, refer to Patent Document 1).

Ferritic 9% Ni steel has been used for a tank for a liquefied naturalgas (LNG) (sometimes referred to as an LNG tank) which is representativeof liquefied gas. Although LNG has a higher temperature than liquidhydrogen, 9% Ni steel has excellent low temperature toughness, and inthe related art, various 9% Ni steels and 7% Ni steels suitable for LNGtanks have been proposed (for example, refer to Patent Documents 2 to4). Moreover, 9% Ni steel can also have a yield strength of 590 MPa ormore at room temperature, and can also be applied to a large LNG tank.

For example, Patent Document 2 discloses a steel for low temperaturewith a plate thickness of 25 mm, which contains 5% to 7.5% of Ni, has ayield stress of more than 590 MPa at room temperature, and a brittlefracture surface ratio of 50% or less in a Charpy test at −233° C. InPatent Document 2, low temperature toughness is secured by setting thevolume fraction of residual austenite stable at −196° C. to 2% to 12%.

In addition, Patent Document 3 discloses a steel for low temperaturewith a plate thickness of 6 mm to 50 mm, which contains 5% to 10% of Ni,has a yield stress of 590 MPa or more at room temperature, and hasexcellent low temperature toughness at −196° C. after strain aging. InPatent Document 3, low temperature toughness after strain aging issecured by setting the volume fraction of residual austenite to 3% ormore and the effective grain size to 5.5 μm or less, and introducingappropriate defects into the intragranular structure.

Furthermore, Patent Document 4 discloses a thin nickel steel plate forlow temperature with a thickness of 6 mm, which contains 7.5% to 12% Niand is excellent in the low temperature toughness of not only the basemetal but also a welded heat-affected zone. In Patent Document 4, the Siand Mn contents are reduced so as not to generate martensite-islandsconstituents in the welded heat-affected zone, whereby low temperaturetoughness at −196° C. is secured.

PRIOR ART DOCUMENT Patent Document

-   [Patent Document 1] Japanese Patent No. 5709881-   [Patent Document 2] Japanese Unexamined Patent Application, First    Publication No. 2014-210948-   [Patent Document 3] Japanese Unexamined Patent Application, First    Publication No. 2011-219849-   [Patent Document 4] Japanese Unexamined Patent Application, First    Publication No. H3-223442

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, the austenitic high Mn stainless steel disclosed in PatentDocument 1 has a larger coefficient of thermal expansion than ferritic9% Ni steel. For large liquid hydrogen tanks, 9% Ni steel with a lowcoefficient of thermal expansion is advantageous due to problems such asfatigue. On the other hand, as a result of examinations by the presentinventors, the 9% Ni steel and 7% Ni steel disclosed in Patent Documents2 to 4 cannot obtain sufficient toughness at −253° C., which is theliquefaction temperature of liquid hydrogen.

The present invention has been made in view of such circumstances, andan object thereof is to provide a nickel-containing steel for lowtemperature, which has sufficient toughness at a low temperature ofaround −253° C., a yield stress of 460 MPa or more at room temperature,and a tensile strength of 560 MPa or more at room temperature.

Means for Solving the Problem

The present inventors conducted numerous examinations on the toughnessat a low temperature of around −253° C. and the tensile strength andyield stress at room temperature of a steel having a higher Ni contentthan 9% Ni steel in the related art. As a result, it was found that inorder to secure low temperature toughness, it is necessary to limit theSi content, strictly limit the Mn content, and optimally controlling thevolume fraction of austenite and the average grain size and averageaspect ratio of prior austenite grains.

The present invention has been made based on the above findings, and thegist thereof is as follows.

(1) According to an aspect of the present invention, a nickel-containingsteel for low temperature includes, as a chemical composition, by mass%: C: 0.030% to 0.070%; Si: 0.03% to 0.30%; Mn: 0.20% to 0.80%; Ni:10.5% to 12.4%; Al: 0.010% to 0.060%; N: 0.0015% to 0.0060%; O: 0.0007%to 0.0030%; Cu: 0% to 0.50%; Cr: 0% to 0.50%; Mo: 0% to 0.40%; Nb: 0% to0.020%; V: 0% to 0.080%; Ti: 0% to 0.020%; B: 0% to 0.0020%; Ca: 0% to0.0040%; REM: 0% to 0.0050%; P: 0.0080% or less; S: 0.0040% or less; anda remainder: Fe and impurities, in which a metallographic structure of athickness middle portion contains 2.0 vol % to 20.0 vol % of anaustenite phase; an average grain size of prior austenite grainsmeasured in a section of the thickness middle portion parallel to arolling direction and a thickness direction is 3.0 μm to 15.0 μm; anaverage aspect ratio of the prior austenite grains measured in thesection of the thickness middle portion parallel to the rollingdirection and the thickness direction is 1.0 to 2.4; a plate thicknessis 4.5 mm to 30 mm; when the plate thickness is more than 20 mm, thenickel-containing steel contains Ni: 11.5% or more; when the platethickness is 20 mm or less and the nickel-containing steel contains Ni:less than 11.5%, the nickel-containing steel contains C: 0.060% or less,Si: 0.19% or less, Mn: 0.30% to 0.50%, Al: 0.050% or less, N: 0.0050% orless, Cr: 0.35% or less, Nb: 0.015% or less, V: 0.060% or less, Ti:0.015% or less, P: 0.0060% or less, and S: 0.0030% or less, and theaverage grain size of the prior austenite grains is 8.0 μm or less; ayield stress at room temperature is 460 MPa to 710 MPa, and a tensilestrength at the room temperature is 560 MPa to 810 MPa.

(2) The nickel-containing steel for low temperature according to (1) mayinclude Ni: 11.5% or more and Mn: 0.50% or less as the chemicalcomposition.

(3) The nickel-containing steel for low temperature according to (1) or(2) may include Ni: 11.5% or more as the chemical composition, in whichthe average grain size of the prior austenite grains may be 9.0 μm orless.

(4) In the nickel-containing steel for low temperature according to anyone of (1) to (3), an average effective grain size measured in thesection of the thickness middle portion parallel to the rollingdirection and the thickness direction may be 2.0 μm to 8.0 μm.

(5) In the nickel-containing steel for low temperature according to anyone of (1) to (3), an average effective grain size measured in thesection of the thickness middle portion parallel to the rollingdirection and the thickness direction may be 2.0 μm to 5.0 μm.

Effects of the Invention

According to the present invention, it is possible to provide anickel-containing steel for low temperature having sufficient extremelylow temperature toughness for uses such as a liquid hydrogen tank, andsufficient yield stress and tensile strength at room temperature.Therefore, for example, when the nickel-containing steel for lowtemperature of the present invention is used in a liquid hydrogen tank,the plate thickness of a steel plate for the tank can be made thinnerthan that of austenitic stainless steel. Therefore, according to thepresent invention, it is possible to achieve an increase in the size anda reduction in the weight of the liquid hydrogen tank, an improvement inheat insulation performance by a reduction in surface area with respectto volume, an effective use of the tank site, an improvement in the fuelefficiency of a liquid hydrogen carrier, and the like. Furthermore,compared to the austenitic stainless steel, the nickel-containing steelfor low temperature of the present invention has a small coefficient ofthermal expansion, so that the design of a large tank can be simplifiedand the tank manufacturing cost can be reduced. As described above, theindustrial contribution of the present invention is extremelyremarkable.

EMBODIMENTS OF THE INVENTION

The toughness of a steel for low temperature in the related art (forexample, 9% Ni steel) was evaluated at −165° C. or −196° C., but atoughness evaluation temperature for a nickel-containing steel for lowtemperature (hereinafter, simply abbreviated to “Ni steel”) according tothe present embodiment is significantly lower than that of the steel inthe related art.

The present inventors conducted numerous examinations in order toclarify the influence of the amounts of elements, a metallographicstructure, and the like on the toughness of Ni steel at −253° C.According to the knowledge in the related art, it has been consideredeffective to increase the Ni content in order to increase lowtemperature toughness. However, as a result of examinations by thepresent inventors, it was found that the toughness at a low temperatureis not sufficiently improved even if the amount of Ni in 9% Ni steel inthe related art is simply changed to increase.

In addition, for the distinction from temperatures such as −165° C. and−196° C. and concise description, hereinafter, a temperature of around−253° C. is referred to as “extremely low temperature” for convenience.

Furthermore, the present inventors examined another method forincreasing the toughness of Ni steel at an extremely low temperature(hereinafter, referred to as “extremely low temperature toughness”). Asa result, it was determined that in order to secure low temperaturetoughness at an extremely low temperature, it is necessary to, inaddition to increase the Ni content, (a) set the C content to 0.030% to0.070%, (b) set the Si content to 0.03% to 0.30%, (c) set the Mn contentto 0.20% to 0.80%, (d) set the P content to 0.0080% or less, (e) controlthe average grain size and average aspect ratio of prior austenitegrains, and (f) control the volume fraction of an austenite phase.Furthermore, the knowledge that the low temperature toughness at anextremely low temperature is further improved by (g) controlling anaverage effective grain size was also obtained. Moreover, it was foundthat in a case where the plate thickness of Ni steel is 20 mm or lessand the above-described conditions are more strictly limited, the Nicontent can be slightly reduced to reduce raw material costs.

Next, a Ni steel according to the present embodiment will be described.

In addition, it is necessary to change the Ni content of the Ni steelaccording to the present embodiment depending on the plate thickness. Ina case where the plate thickness is large (that is, the plate thicknessis more than 20 mm), the cooling rate during reheating hardening becomesslow, and it becomes difficult to secure the low temperature toughnessthrough heat treatments. Therefore, in a case where the plate thicknessis more than 20 mm, the amount of Ni, which is an element for securingthe low temperature toughness, has to be 11.5% or more. On the otherhand, in a case where the plate thickness is 20 mm or less, it is easyto secure low temperature toughness through heat treatments, so that itis possible to impart sufficient low temperature toughness to the Nisteel while suppressing the Ni content to less than 11.5%. As a matterof course, the Ni steel according to the present embodiment may have aplate thickness of 20 mm or less and a Ni content of 11.5% or more.

However, in a case where the plate thickness is 20 mm or less and Ni isless than 11.5% (hereinafter, sometimes abbreviated to “in a case wherethe Ni content is low”), it is necessary to more strictly controlelements that affect the low temperature toughness other than the Nicontent (the amounts of C, Si, Mn, Al, N, Cr, Nb, V, Ti, P, and S, andthe average grain size of prior austenite grains) compared to the casewhere Ni content is 11.5% or more.

Regarding the requirements that require further limitation depending onthe Ni content and the plate thickness due to the above circumstances,the intent thereof will be described as appropriate.

First, the composition of the Ni steel according to the presentembodiment will be described. Unless otherwise specified, % in contentsmeans mass %.

(C: 0.030% to 0.070%)

C is an element that increases the yield stress at room temperature, andalso contributes to the formation of martensite and austenite. When theC content is less than 0.030%, strength cannot be secured, and theextremely low temperature toughness of the Ni steel may decrease due tothe formation of coarse bainite. Therefore, the lower limit of the Ccontent of the Ni steel is set to 0.030%. A preferable lower limit ofthe C content is 0.035%.

On the other hand, when the C content exceeds 0.070%, cementite islikely to precipitate at prior austenite grain boundaries, and thiscementite causes fracture at the grain boundaries, thereby reducing theextremely low temperature toughness of the Ni steel. Therefore, theupper limit of the C content is set to 0.070%. The upper limit of the Ccontent is preferably 0.060%, more preferably 0.050%, and even morepreferably 0.045%.

In a case where the Ni content is small, the C content needs to be0.060% or less. In a case where the Ni content is small, a preferableupper limit of the C content is 0.055%, 0.050%, or 0.045%. The lowerlimit and preferable lower limit of the C content of a Ni steel with asmall Ni content may be the same as those of a Ni steel having a Nicontent of 11.5% or more.

(Si: 0.03% to 0.30%) Si is an element that increases the yield stress atroom temperature. When the Si content is less than 0.03%, the effect ofimproving the yield stress at room temperature is small. Therefore, thelower limit of the Si content of the Ni steel is set to 0.03%. Apreferable lower limit of the Si content is 0.05%.

On the other hand, when the Si content exceeds 0.30%, cementite at theprior austenite grain boundaries is likely to be coarsened, and thiscementite causes fracture at the grain boundaries, thereby reducing theextremely low temperature toughness of the Ni steel. Therefore, limitingthe upper limit of the Si content to 0.30% is extremely important inorder to secure the toughness at an extremely low temperature. The upperlimit of the Si content is preferably 0.20% and more preferably 0.15%,and the upper limit of the Si content is even more preferably 0.10%.

In a case where the Ni content is small, the Si content needs to be0.19% or less. In a case where the Ni content is small, a preferableupper limit of the Si content is 0.16%, 0.13%, or 0.10% or less. Thelower limit and preferable lower limit of the Si content of a Ni steelwith a small Ni content may be the same as those of a Ni steel having aNi content of 11.5% or more.

(Mn: 0.20% to 0.80%)

Mn is an element that increases the yield stress at room temperature.When the Mn content is less than 0.20%, strength cannot be secured, andextremely low temperature toughness may decrease due to the formation ofcoarse bainite and the like. Therefore, the lower limit of the Mncontent is set to 0.20%. A preferable lower limit of the Mn content is0.30%, 0.28%, or 0.25%.

On the other hand, when the Mn content exceeds 0.80%, Mn segregated atthe prior austenite grain boundaries and MnS precipitated coarsely causefractures at the grain boundaries, resulting in a decrease in theextremely low temperature toughness. Therefore, limiting the upper limitof the Mn content to 0.80% is extremely important in order to secure thetoughness at an extremely low temperature. The upper limit of the Mncontent is preferably 0.60%, and more preferably 0.50%.

In a case where the Ni content is small, the Mn content needs to be0.30% to 0.50%. In a case where the Ni content is small, a preferablelower limit of the Mn content is 0.35% or more. In a case where the Nicontent is small, a preferable upper limit of the Mn content is 0.45% or0.40%.

(Ni: 10.5% to 12.4%)

Ni is an essential element for securing the extremely low temperaturetoughness. When the Ni content is less than 10.5%, the toughness at anextremely low temperature is insufficient. Therefore, the lower limit ofthe Ni content is set to 10.5%. A preferable lower limit of the Nicontent is 10.8%, 11.0%, or 11.5%. On the other hand, Ni is an expensiveelement, and when Ni is contained in more than 12.4%, the economy isimpaired. Therefore, the Ni content is limited to 12.4% or less. Theupper limit of the Ni content may be set to 12.2%, 12.0%, or 11.8%. In acase where the plate thickness is 20 mm or less, the upper limit of theNi content may be set to 11.3%, 11.1%, or 10.9%.

In a case where the plate thickness is more than 20 mm, the Ni contentneeds to be 11.5% or more. In a case where the plate thickness is morethan 20 mm, a preferable lower limit of the Ni content is 11.8% or12.0%. The upper limit and preferable upper limit of the Ni content of aNi steel having a plate thickness of more than 20 mm may be the samevalues as those of a Ni steel having a plate thickness of 20 mm or less.

(Al: 0.010% to 0.060%)

Al is an element mainly used for deoxidation. In addition, Al forms AlNand contributes to the refinement of the metallographic structure and areduction in the amount of solute N, which lowers the extremely lowtemperature toughness. When the Al content is less than 0.010%, theeffect of deoxidation, the effect of the refinement of themetallographic structure, and the effect of reducing the amount ofsolute N are small. Therefore, the lower limit of the Al content is setto 0.010% or less. The lower limit of the Al content is preferably0.015% or more, and more preferably 0.020% or more.

However, when the Al content exceeds 0.060%, the toughness at anextremely low temperature decreases. Therefore, the upper limit of theAl content is set to 0.060%. A more preferable upper limit of the Alcontent is 0.040% or 0.035%.

In a case where the Ni content is small, the Al content needs to be0.050% or less. In a case where the Ni content is small, a preferableupper limit of the Al content is 0.040% or 0.020%. The lower limit andpreferable lower limit of the Al content of a Ni steel with a small Nicontent may be the same as those of a Ni steel having a Ni content of11.5% or more.

(N: 0.0015% to 0.0060%)

N contributes to the formation of nitrides that refine grains. When theN content is reduced to less than 0.0015%, fine AlN that suppresses thecoarsening of the austenite grain size during a heat treatment isinsufficient, and there are cases where the austenite grains becomecoarse and the extremely low temperature toughness of the Ni steeldecreases. For this reason, the N content may be set to 0.0015% or more,and preferably 0.0020% or more, or 0.0025% or more.

On the other hand, when the N content exceeds 0.0060%, the amount ofsolute N increases or coarsening of AlN occurs, resulting in thedecrease in the toughness at an extremely low temperature. For thisreason, the N content is set to 0.0060% or less, preferably 0.0050% orless, and more preferably 0.0040% or less or 0.0035% or less.

In a case where the Ni content is small, the N content needs to be0.0050% or less. In a case where the Ni content is small, a preferableupper limit of the N content is 0.0040% or 0.0030%. The lower limit andpreferable lower limit of the N content of a Ni steel with a small Nicontent may be the same as those of a Ni steel having a Ni content of11.5% or more.

(O: 0.0007% to 0.0030%)

O is an impurity, and when the O content exceeds 0.0030%, there arecases where Al₂O₃ clusters increase and the toughness at an extremelylow temperature decreases. Therefore, the upper limit of the O contentis set to 0.0030%. The upper limit of the O content is preferably0.0025%, more preferably 0.0020%, and even more preferably 0.0015%.Although it is desirable that the O content is small, there are caseswhere a reduction in the O content to less than 0.0007% is accompaniedby an increase in cost. Therefore, the lower limit of the 0 content isset to 0.0007%. The lower limit of the O content may be set to 0.0008%or 0.0010%. The upper and lower limits and preferable upper and lowerlimits of the O content are the above values regardless of the platethickness and the Ni content.

(P: 0.0080% or Less)

P is an element that causes grain boundary embrittlement at the prioraustenite grain boundaries and is thus harmful to the extremely lowtemperature toughness. Therefore, it is desirable that the P content issmall. When the P content exceeds 0.0080%, there are cases where thetoughness at an extremely low temperature decreases. Therefore, the Pcontent is limited to 0.0080% or less. The upper limit of the P contentis preferably 0.0060%, more preferably 0.0040%, and even more preferably0.0030%.

There are cases where P is incorporated into molten steel as an impurityduring the manufacturing of the molten steel. However, the lower limitthereof does not need to be particularly limited, and the lower limitthereof is 0%. However, when the P content is excessively reduced, thereare cases where the manufacturing cost increases. Therefore, the lowerlimit of the P content may be 0.0002%, 0.0005%, or 0.0008%.

In a case where the Ni content is small, the P content needs to be0.0060% or less. In a case where the Ni content is small, a preferableupper limit of the P content is 0.0050%, 0.0040%, or 0.0030%. The lowerlimit and preferable lower limit of the P content of a Ni steel with asmall Ni content may be the same as those of a Ni steel having a Nicontent of 11.5% or more.

(S: 0.0040% or Less)

S forms MnS, which becomes a brittle fracture origin in some cases, andis thus an element harmful to the extremely low temperature toughness.When the S content exceeds 0.0040%, there are cases where the toughnessat an extremely low temperature decreases. Therefore, the S content islimited to 0.0040% or less. The upper limit of the S content ispreferably 0.0030%, more preferably 0.0020%, and even more preferably0.0010%.

There are cases where S is incorporated into molten steel as an impurityduring the manufacturing of the molten steel. However, the lower limitthereof does not need to be particularly limited, and the lower limitthereof is 0%. However, when the S content is excessively reduced, thereare cases where the manufacturing cost increases. Therefore, the lowerlimit of the S content may be 0.0002%, 0.0005%, or 0.0008%.

In a case where the Ni content is small, the S content needs to be0.0030% or less. In a case where the Ni content is small, a preferableupper limit of the S content is 0.0010%, 0.0015%, or 0.0010%. The lowerlimit and preferable lower limit of the S content of a Ni steel with asmall Ni content may be the same as those of a Ni steel having a Nicontent of 11.5% or more.

(Cu: 0% to 0.50%)

Cu is an element that increases the yield stress at room temperature, sothat the Ni steel according to the present embodiment may contain Cu.However, when the Cu content exceeds 0.50%, the toughness at anextremely low temperature decreases. Therefore, the upper limit of theCu content is set to 0.50%. The upper limit of the Cu content ispreferably 0.40%, more preferably 0.30%, and even more preferably 0.20%.

There are cases where Cu is incorporated as an impurity into the Nisteel during the manufacturing of molten steel. However, the lower limitthereof does not need to be particularly limited, and the lower limitthereof is 0%. The lower limit of the Cu content may be set to 0.02%,0.05%, or 0.10%. The upper and lower limits and preferable upper andlower limits of the Cu content are the above values regardless of theplate thickness and the Ni content.

(Cr: 0% to 0.50%)

Cr is an element that increases the yield stress at room temperature, sothat the Ni steel according to the present embodiment may contain Cr.However, when the Cr content exceeds 0.50%, the toughness at anextremely low temperature decreases. Therefore, the upper limit of theCr content is set to 0.35%. The upper limit of the Cr content ispreferably 0.30%, more preferably 0.20%, and even more preferably 0.10%.

There are cases where Cr is incorporated as an impurity into the Nisteel during the manufacturing of molten steel. However, the lower limitthereof does not need to be particularly limited, and the lower limitthereof is 0%. The lower limit of the Cr content may be set to 0.02%,0.05%, or 0.10%.

In a case where the Ni content is small, the Cr content needs to be0.35% or less. In a case where the Ni content is small, a preferableupper limit of the Cr content is 0.30%, 0.25%, or 0.20%. The lower limitand preferable lower limit of the Cr content of a Ni steel with a smallNi content may be the same as those of a Ni steel having a Ni content of11.5% or more.

(Mo: 0% to 0.40%)

Mo is an element that increases the yield stress at room temperature andalso has an effect of suppressing grain boundary embrittlement, so thatthe Ni steel according to the present embodiment may contain Mo.However, Mo is an expensive element, and when the Mo content exceeds0.40%, the economy is impaired. Therefore, the Mo content is limited to0.40% or less.

There are cases where Mo is incorporated as an impurity into the Nisteel during the manufacturing of molten steel. However, the lower limitthereof does not need to be particularly limited, and the lower limitthereof is 0%. The lower limit of the Mo content may be set to 0.02%,0.05%, or 0.10%. The upper and lower limits and preferable upper andlower limits of the Mo content are the above values regardless of theplate thickness and the Ni content.

(Nb: 0% to 0.020%)

Nb is an element that increases the yield stress at room temperature,and also has an effect of improving the extremely low temperaturetoughness by refining the metallographic structure, so that the Ni steelaccording to the present embodiment may contain Nb. However, when the Nbcontent exceeds 0.020%, the toughness at an extremely low temperaturedecreases. Therefore, the upper limit of the Nb content is set to0.020%. The upper limit of the Nb content is preferably 0.015%, and morepreferably 0.010%.

There are cases where Nb is incorporated as an impurity into the Nisteel during the manufacturing of molten steel. However, the lower limitthereof does not need to be particularly limited, and the lower limitthereof is 0%. The lower limit of the Nb content may be set to 0.002%,or 0.005%

In a case where the Ni content is small, the Nb content needs to be0.015% or less. In a case where the Ni content is small, a preferableupper limit of the Nb content is 0.012% or 0.010%. The lower limit andpreferable lower limit of the Nb content of a Ni steel with a small Nicontent may be the same as those of a Ni steel having a Ni content of11.5% or more.

(V: 0% to 0.080%)

V is an element that increases the yield stress at room temperature, sothat the Ni steel according to the present embodiment may contain V.However, when the V content exceeds 0.080%, the toughness at anextremely low temperature decreases. Therefore, the upper limit of the Vcontent is set to 0.080%. The upper limit of the V content is preferably0.060%, and more preferably 0.040%.

There are cases where V is incorporated as an impurity into the Ni steelduring the manufacturing of molten steel. However, the lower limitthereof does not need to be particularly limited, and the lower limitthereof is 0%. The lower limit of the V content may be set to 0.002%,0.005%, or 0.010%.

In a case where the Ni content is small, the V content needs to be0.060% or less. In a case where the Ni content is small, a preferableupper limit of the V content is 0.050% or 0.040%. The lower limit andpreferable lower limit of the V content of a Ni steel with a small Nicontent may be the same as those of a Ni steel having a Ni content of11.5% or more.

(Ti: 0% to 0.020%)

Ti forms TiN and contributes to the refinement of the metallographicstructure and a reduction in the amount of solute N that lowers theextremely low temperature toughness, so that the Ni steel according tothe present embodiment may contain Ti. However, when the Ti contentexceeds 0.020%, the toughness at an extremely low temperature decreases.Therefore, the upper limit of the Ti content is set to 0.020%. The upperlimit of the Ti content is preferably 0.015%, and more preferably0.010%.

There are cases where Ti is incorporated as an impurity into the Nisteel during the manufacturing of molten steel. However, the lower limitthereof does not need to be particularly limited, and the lower limitthereof is 0%. The lower limit of the Ti content may be set to 0.001%,0.002%, or 0.005%.

In a case where the Ni content is small, the Ti content needs to be0.015% or less. In a case where the Ni content is small, a preferableupper limit of the Ti content is 0.012% or 0.010%. The lower limit andpreferable lower limit of the Ti content of a Ni steel with a small Nicontent may be the same as those of a Ni steel having a Ni content of11.5% or more.

The upper and lower limits and preferable upper and lower limits of theamounts of B, Ca, REM, Sb, Sn, As, Co, Zn, and W described below are thesame regardless of the plate thickness and Ni content.

(B: 0% to 0.0020%)

B is an element that increases the yield stress at room temperature, andalso contributes to a reduction in the amount of solute N, which lowersthe extremely low temperature toughness, by forming BN. Therefore, theNi steel according to the present embodiment may contain B. However,when B is contained in more than 0.0020%, the toughness at an extremelylow temperatures decreases, so that the upper limit of the B content isset to 0.0020%. The upper limit of the B content is preferably 0.0015%,more preferably 0.0012%, and even more preferably 0.0010%.

There are cases where B is incorporated as an impurity into the Ni steelduring the manufacturing of molten steel. However, the lower limitthereof does not need to be particularly limited, and the lower limitthereof is 0%. The lower limit of the B content may be set to 0.0001%,0.0002%, or 0.0005%.

(Ca: 0% to 0.0040%)

Ca causes MnS, which is an inclusion that tends to be stretched by hotrolling and thus easily increases the harmfulness to the extremely lowtemperature toughness, to be spheroidized as CaS, thereby beingeffective in improving the extremely low temperature toughness.Therefore, the Ni steel according to the present embodiment may containCa. However, when the Ca content exceeds 0.0040%, oxysulfides containingCa are coarsened, and these oxysulfides lower the toughness of the Nisteel at an extremely low temperature. Therefore, the upper limit of theCa content is limited to 0.0040%, and is preferably set to 0.0030%.

There are cases where Ca is incorporated as an impurity into the Nisteel during the manufacturing of molten steel. However, the lower limitthereof does not need to be particularly limited, and the lower limitthereof is 0%. The lower limit of the Ca content may be set to 0.0005%,0.0010%, or 0.0015%.

(REM: 0% to 0.0050%)

REM (rare-earth metal element) refers to a total of 17 elements composedof Sc, Y, and lanthanoids, and the amount of REM means the total amountof these 17 elements. Like Ca, REM causes MnS, which is an inclusionthat tends to be stretched by hot rolling and thus easily increases theharmfulness to the extremely low temperature toughness, to bespheroidized as an oxysulfide of REM, thereby being effective inimproving the extremely low temperature toughness. Therefore, the Nisteel according to the present embodiment may contain REM. However, whenthe REM content exceeds 0.0050%, oxysulfides containing REM arecoarsened, and these oxysulfides lower the toughness of the Ni steel atan extremely low temperature. Therefore, the upper limit of the REMcontent is limited to 0.0050%, and is preferably set to 0.0040%.

There are cases where REM is incorporated as an impurity into the Nisteel during the manufacturing of molten steel. However, the lower limitthereof does not need to be particularly limited, and the lower limitthereof is 0%. The lower limit of the REM content may be set to 0.0005%,0.0010%, or 0.0015%.

The Ni steel according to the present embodiment contains or limits theabove-mentioned elements, and the remainder consists of iron andimpurities. Here, the impurities mean elements that are incorporatedinto the Ni steel due to various factors in the manufacturing process,including raw materials such as ore and scrap, when steel isindustrially manufactured, and are allowed in a range in which the Nisteel according to the present embodiment is not adversely affected.However, in the Ni steel according to the present embodiment, it isnecessary to define the upper limits of P and S among the impurities asdescribed above. In addition to the above-mentioned elements, the Nisteel according to the present embodiment may contain, for example, thefollowing alloying elements for the purpose of further improving thestrength of the steel material itself, the extremely low temperaturetoughness, and the like, or as impurities from auxiliary raw materialssuch as scrap.

Sb impairs the extremely low temperature toughness. Therefore, the Sbcontent is preferably 0.005% or less, more preferably 0.003% or less,and most preferably 0.001% or less.

Sn impairs the extremely low temperature toughness. Therefore, the Sncontent is preferably 0.005% or less, more preferably 0.003% or less,and most preferably 0.001% or less.

As impairs the extremely low temperature toughness. Therefore, the Ascontent is preferably 0.005% or less, more preferably 0.003% or less,and most preferably 0.001% or less.

Moreover, in order to fully exhibit the effects of the above-describedelements, it is preferable to limit the amount of each of Co, Zn, and Wto 0.01% or less or 0.005% or less.

There is no need to limit the lower limits of Sb, Sn, As, Co, Zn, and W,and the lower limit of each of the elements is 0%. Moreover, even if analloying element (for example, P, S, Cu, Cr, Mo, Nb, V, Ti, B, Ca, andREM) with no defined lower limit or with a lower limit of 0% isintentionally added or incorporated as an impurity, when the amountthereof is within the above-described range, the steel (steel material)for low temperature is interpreted as the Ni steel according to thepresent embodiment.

Next, the metallographic structure of the Ni steel according to thepresent embodiment will be described. In addition, regarding therequirements that require further limitation depending on the Ni contentand the plate thickness, the intent thereof will be described asappropriate.

The present inventors newly found that fracture occurs at the prioraustenite grain boundaries at an extremely low temperature, andtoughness is likely to decrease. The Ni steel according to the presentembodiment is manufactured by performing water cooling or air coolingafter hot rolling and performing heat treatments including reheatinghardening, an intermediate heat treatment, and tempering. The prioraustenite grain boundaries mentioned here are grain boundaries ofaustenite that have existed during heating of the reheating hardening. Alarge proportion of prior austenite grain boundaries that have existedduring the heating of the reheating hardening are coarse. It isconsidered that Mn, P, and Si are segregated at the coarse prioraustenite grain boundaries, and these elements lower the bonding forceof the prior austenite grain boundaries and impair the toughness at−253° C.

Although prior austenite grain boundaries are newly generated during theintermediate heat treatment, in the manufacturing of the Ni steelaccording to the present embodiment, the temperature of the intermediateheat treatment is as low as 610° C. to 650° C., and there are very fewcoarse prior austenite grains newly generated. Since the amount of Mn,P, and Si segregated at prior austenite grain boundaries which are notcoarse is relatively small, fracture from the prior austenite grainboundaries (most of which are prior austenite grain boundaries generatedduring the intermediate heat treatment) which are not coarse isrelatively unlikely to occur.

Therefore, in order to secure the extremely low temperature toughness,it is substantially important to control the coarse prior austenitegrains. Therefore, in the Ni steel according to the present embodiment,in a case of measuring the grain size or aspect ratio of the prioraustenite grains, only coarse prior austenite grains are measurementobjects. In the present embodiment, whether or not the prior austenitegrain boundaries are coarse is determined based on whether or not thegrain size of the prior austenite grains is 2.0 μm or more. That is, theprior austenite grains having a grain size of less than 2.0 μm aredetermined to be prior austenite grains that do not impair the extremelylow temperature toughness. When the average grain size or average aspectratio of the prior austenite grains is measured, the prior austenitegrains having a grain size of less than 2.0 μm are excluded. In the Nisteel according to the present embodiment, the “average grain size ofthe prior austenite grains” means the average value of the grain sizesof prior austenite grains having a grain size of 2.0 μm or more, and the“average aspect ratio of the prior austenite grains” means the averagevalue of the aspect ratios of prior austenite grains having a grain sizeof 2.0 μm or more.

The present inventors conducted numerous examinations on methods forsuppressing fracture at the prior austenite grain boundaries at anextremely low temperature. As a result, the present inventors found thatfracture at the prior austenite grain boundaries is suppressed and thetoughness at an extremely low temperature can be secured when sixconditions are simultaneously satisfied including (A) setting the Ccontent to 0.070% or less (corresponding to a case where the Ni contentis 11.5% or more, and the same applies to (B) to (F)), (B) setting theMn content to 0.80% or less, (C) setting the P content to 0.0080% orless, (D) setting the Si content to 0.30% or less, (E) causing theaverage grain size of the prior austenite grains to be 15.0 μm or less,and (F) suppressing the average aspect ratio of the prior austenitegrains to 2.4 or less. In addition, when the plate thickness is causedto be 20 mm or less and the conditions (A) to (F) are more strictlycontrolled, the Ni content can be reduced to less than 11.5%, so thatraw material costs can be reduced.

As described above, it is presumed that at an extremely low temperature,fracture is likely to occur selectively in a portion where the bondingforce is relatively weak, such as a coarse prior austenite grainboundary. Therefore, it is considered that the decrease in the bondingforce of the coarse prior austenite grain boundaries can be suppressedby suppressing precipitation of cementite and segregation of Mn and Pthat weakens the bonding force of the coarse prior austenite grainboundaries. Moreover, an increase in the C content and the Si contentand coarsening of the prior austenite grains promote the coarsening ofintergranular cementite. Therefore, the suppression of the C content andthe Si content and the refinement of the prior austenite are necessaryfor suppressing the fracture at the prior austenite grain boundaries atan extremely low temperature.

(Average Grain Size of Prior Austenite Grains Measured in Section ofThickness Middle Portion Parallel to Rolling Direction and Thicknessdirection: 3.0 μm to 15.0 μm)

The average grain size of the prior austenite grains measured in asection of a thickness middle portion parallel to a rolling directionand a thickness direction needs to be 3.0 μm to 15.0 μm. In the presentembodiment, unless otherwise specified, the average grain size of theprior austenite grains indicates an average grain size measured in thesection of the thickness middle portion parallel to the rollingdirection and the thickness direction. When the average grain size ofthe prior austenite grains is more than 15.0 μm, cementite precipitatedat the prior austenite grain boundaries becomes coarse, and theconcentration of Mn and P at the grain boundaries increases.Precipitation of coarse cementite and concentration of Mn and P weakenthe bonding force of the prior austenite grain boundaries and causefractures at the prior austenite grain boundaries in some cases. Inaddition, there are cases where segregation points of Mn and P andcoarse cementite become brittle fracture origins. As described above,since an increase in the average grain size of the prior austenitegrains decreases the toughness at an extremely low temperature, theupper limit of the average grain size of the prior austenite grains isset to 15.0 μm. The upper limit of the average grain size of the prioraustenite grains may be set to 12.0 μm, 10.0 μm, 9.0 μm, 8.0 μm, or 7.5μm. In order to refine the average grain size of the prior austenitegrains to less than 3.0 μm, methods accompanied by an increase inmanufacturing cost such as an increase in the number of heat treatmentsare necessary. Therefore, the lower limit of the average grain size ofthe prior austenite grains is set to 3 μm.

In a case where the Ni content is small, the average grain size of theprior austenite grains needs to be 8.0 μm or less. The upper limitthereof may be set to 7.0 μm or 6.0 μm as necessary. The lower limit ofthe average grain size of the prior austenite grains of a Ni steelhaving a small Ni content may be the same as that of a Ni steel having aNi content of 11.5% or more.

(Average Aspect Ratio of Prior Austenite Grains Measured in Section ofThickness Middle Portion Parallel to Rolling Direction and ThicknessDirection: 1.0 to 2.4)

Furthermore, in the Ni steel according to the present embodiment, theaverage aspect ratio of the prior austenite grains is 2.4 or less. Inthe present embodiment, unless otherwise specified, the average aspectratio of the prior austenite grains indicates an average aspect ratiomeasured in the section of the thickness middle portion parallel to therolling direction and the thickness direction. The average aspect ratioof the prior austenite grains is the ratio between the length andthickness of the prior austenite grains in the section (L-section)parallel to the rolling direction and the thickness direction, that is,the length of the prior austenite grains in the rolling direction/thethickness of the prior austenite grains in the thickness direction.Therefore, the lower limit of the average aspect ratio is in a casewhere the length and thickness of the prior austenite grains are thesame, and the average aspect ratio is 1.0 or more.

The above numerical value range of the average aspect ratio of the prioraustenite grains is achieved in a case where a manufacturing method,which will be described below, is applied to the steel having theabove-described chemical composition. The upper limit of the averageaspect ratio of the prior austenite grains may be set to 2.3, 2.2, 2.0,1.8, or 1.7. The upper and lower limits and preferable upper and lowerlimits of the average aspect ratio of the prior austenite grains are theabove values regardless of the plate thickness and the Ni content.

The average grain size and the average aspect ratio of the prioraustenite grains are measured using the section (L-section) of thethickness middle portion parallel to the rolling direction and thethickness direction as an observed section. The prior austenite grainboundaries are revealed by corroding the observed section with asaturated aqueous solution of picric acid. An enlarged photograph of theL-section subjected to the corrosion treatment is photographed at fiveor more visual fields with a scanning electron microscope (SEM) at amagnification of 1,000-fold or 2,000-fold. The circle equivalent grainsizes (diameters) of at least 20 prior austenite grains having a circleequivalent grain size (diameter) of 2.0 μm or more, which are includedin these SEM photographs, are obtained by image processing, and theaverage value thereof is calculated, thereby obtaining the average grainsize of the prior austenite grains measured in the section of thethickness middle portion parallel to the rolling direction and thethickness direction. In addition, the ratios (aspect ratios) between thelength in the rolling direction and the thickness in the thicknessdirection of at least 20 prior austenite grains having a circleequivalent grain size (diameter) of 2.0 μm or more, which are includedin the above SEM photographs, are measured, and the average valuethereof is calculated, thereby obtaining the average aspect ratio of theprior austenite grains measured in the section of the thickness middleportion parallel to the rolling direction and the thickness direction.In a case where prior austenite grains having a grain size of less than2.0 μm are included, the above-described measurement is performedexcluding the prior austenite grains.

(Volume Fraction of Austenite Phase in Metallographic Structure ofThickness Middle Portion: 2.0 vol % to 20.0 vol %)

In order to increase the toughness at an extremely low temperature, themetallographic structure of the thickness middle portion of the Ni steelat room temperature contains an austenite phase in a volume fraction of2.0 vol % or more. In the present embodiment, unless otherwisespecified, the volume fraction of the austenite phase indicates a volumefraction measured at the thickness middle portion. This austenite phaseis different from prior austenite and is an austenite phase present in aNi steel after a heat treatment, and the volume fraction thereof ismeasured by an X-ray diffraction method. In a case where 2.0 vol % to20.0 vol % of the austenite phase is contained in the thickness middleportion of the Ni steel at room temperature, it is considered that astable austenite phase is present in the Ni steel in an amount that isnecessary for securing the toughness at an extremely low temperatureeven when cooled to an extremely low temperature.

It is considered that in a case where an austenite phase which is stableeven at an extremely low temperature is present, applied stress andstrain are relieved by the plastic deformation of austenite, and thustoughness is improved. In addition, the austenite phase is relativelyuniformly and finely generated at the prior austenite grain boundaries,the block boundaries and lath boundaries of tempered martensite, and thelike. That is, it is considered that the austenite phase is present inthe vicinity of a hard phase, which is likely to be a brittle fractureorigin, relieves the concentration of stress or strain around the hardphase, and thus contributes to the suppression of the occurrence ofbrittle fracture. Furthermore, it is considered that as a result ofgenerating 2.0 vol % or more of the austenite phase in the thicknessmiddle portion, coarse cementite, which becomes a brittle fractureorigin, can be significantly reduced. The lower limit of the volumefraction of the austenite phase in the metallographic structure of thethickness middle portion may be set to 3.0 vol % or 4.0 vol %.

On the other hand, when the volume fraction of the austenite phaseincreases excessively, the concentration of C or the like into theaustenite phase becomes insufficient, and the possibility oftransformation of the austenite phase into martensite at an extremelylow temperature increases. There are cases where unstable austenite thattransforms into martensite at an extremely low temperature reduces theextremely low temperature toughness at an extremely low temperature.Therefore, the volume fraction of the austenite phase in themetallographic structure of the thickness middle portion is preferably20.0 vol % or less, or 15.0 vol % or less. The upper limit of the volumefraction of the austenite phase in the metallographic structure of thethickness middle portion may be set to 12.0 vol %, 10.0 vol %, or 6.0vol %.

In a case where the Ni content is small, the volume fraction of theaustenite phase is preferably set to 6.0 vol % or less. As necessary,the upper limit thereof may be set to 5.0 vol %, 4.5 vol %, or 4.0 vol%.

The remainder of the metallographic structure of the Ni steel accordingto the present embodiment is mainly (tempered) martensite. In order tomanufacture a Ni steel in which the prior austenite grain size and theaverage aspect ratio of prior austenite grains are within theabove-described ranges, it is necessary for the manufacturing method toinclude water cooling or air cooling after hot rolling, reheatinghardening, an intermediate heat treatment, and tempering. In a casewhere such a manufacturing method is applied to a steel having theabove-described chemical composition, the remainder of the obtainedmetallographic structure (that is, the primary phase) is necessarilytempered martensite. In addition, there are cases where the remainder ofthe metallographic structure contains a phase (for example, coarseinclusions) which is not classified as either austenite or temperedmartensite. In a case where the total volume fraction of the austenitephase and the tempered martensite phase in the metallographic structureof the thickness middle portion is 99.0% or more, the inclusion ofphases other than these is allowed. In a case of measuring the volumefraction of the tempered martensite phase, the area fraction measured bymicrostructure observation with an optical microscope using nital as acorrosive solution is used as the volume fraction as it is (this isbecause the area fraction is basically the same as the volume fraction).

The volume fraction of the austenite phase in the thickness middleportion is measured by taking a sample having a surface parallel to theplate surface of the Ni steel from the thickness middle portion of theNi steel and applying an X-ray diffraction method to the sample. Thevolume fraction of the austenite phase is obtained from the ratiobetween the integrated intensities of austenite (face-centered cubicstructure) and tempered martensite (body-centered cubic structure) ofX-ray peaks. Specifically, the taken sample is subjected to X-raydiffraction, and the volume fraction of the austenite phase may bemeasured from the ratio between the integrated intensities of the (111)plane, (200) plane, and (211) plane of an a phase having a BCC structureand the integrated intensities of the (111) plane, (200) plane, and(220) plane of an austenite phase having a FCC structure.

In the present invention, a treatment (so-called deep cooling treatment)for cooling a test piece to an extremely low temperature is unnecessarybefore the measurement of the volume fraction of the austenite phase.However, in a case where only a test piece after being subjected to adeep cooling treatment is present, the volume fraction of the austenitephase may be measured using the test piece after being subjected to thedeep cooling treatment.

(Average Effective Grain Size Measured in Section of Thickness MiddlePortion Parallel to Rolling Direction and Thickness Direction:Preferably 2.0 μm to 8.0 μm)

An average effective grain size measured in the section of the thicknessmiddle portion parallel to the rolling direction and the thicknessdirection (hereinafter abbreviated to “average effective grain size”) ispreferably set to 2.0 μm to 8.0 μm. In the present embodiment, aneffective grain size is defined as the circle equivalent diameter of aregion (effective grain) surrounded by a boundary of a metallographicstructure having an orientation difference of 15° or more. In thepresent embodiment, unless otherwise specified, the average effectivegrain size indicates an average effective grain size measured in thesection of the thickness middle portion parallel to the rollingdirection and the thickness direction. When the effective grain size isrefined, resistance to propagation of fracture cracks increases and thetoughness is improved. However, in order to refine the average effectivegrain size to less than 2.0 μm, methods accompanied by an increase inmanufacturing cost such as an increase in the number of heat treatmentsare necessary. Therefore, the lower limit of the average effective grainsize is preferably set to 2.0 μm. The lower limit of the averageeffective grain size may be set to 3.0 μm, 4.0 μm, or 5.0 μm. On theother hand, when the average effective grain size is more than 8.0 μm,there are cases where stress exerted on hard phases that become thebrittle fracture origins, that is, inclusions such as coarse cementite,coarse AlN, MnS, and alumina in the prior austenite grain boundaries andtempered martensite increases, and the toughness at an extremely lowtemperature decreases. Therefore, the upper limit of the averageeffective grain size is preferably set to 8.0 μm. The upper limit of theaverage effective grain size may be set to 7.0 μm, 6.0 μm, or 5.0 μm.

In a case where the Ni content is small, the average effective grainsize is preferably set to 5.0 μm or less. The lower limit and preferablelower limit of the average effective grain size of a Ni steel having asmall Ni content may be the same as those of a Ni steel having a Nicontent of 11.5% or more.

The average effective grain size is measured by using an electronbackscatter diffraction (EBSD) analyzer attached to a scanning electronmicroscope, with the section (L-section) of the thickness middle portionparallel to the rolling direction and the thickness direction as anobserved section. Observation of five or more visual fields is performedat a magnification of 2,000-fold, a boundary of a metallographicstructure having an orientation difference of 15° or more is regarded asa grain boundary, grains surrounded by the grain boundaries are regardedas effective grains, the circle equivalent grain sizes (diameters) ofthe effective grains are obtained by image processing, and the averagevalue of the circle equivalent grain sizes are calculated, therebyobtaining the average effective grain size measured in the section ofthe thickness middle portion parallel to the rolling direction and thethickness direction.

(Plate Thickness: 4.5 mm to 30 mm)

The Ni steel according to the present embodiment is mainly a Ni steelplate, and the plate thickness thereof is 30 mm or less. A Ni steel witha plate thickness of less than 4.5 mm is rarely used as a material for alarge scale structure such as a liquid hydrogen tank, so that the lowerlimit of the plate thickness is set to 4.5 mm. In a case where the platethickness is more than 30 mm, the cooling rate during the reheatinghardening is extremely slow, and it is very difficult to secure the lowtemperature toughness in the compositional range of the Ni steelaccording to the present application (particularly, the Ni content). Asnecessary, the lower limit of the plate thickness may be set to 6 mm, 8mm, 10 mm, or 12 mm, and the upper limit of the plate thickness may beset to 25 mm, 20 mm, or 16 mm.

(Yield Stress at Room Temperature: 460 MPa to 710 MPa)

(Tensile Strength at Room Temperature: 560 MPa to 810 MPa)

The yield stress of the Ni steel according to the present embodiment atroom temperature is set to 460 MPa to 710 MPa. In addition, the tensilestrength of the Ni steel according to the present embodiment at roomtemperature is set to 560 MPa to 810 MPa. The lower limit of the yieldstress may be set to 470 MPa, 500 MPa, or 520 MPa. The upper limit ofthe yield stress may be set to 690 MPa, 670 MPa, or 650 MPa. The lowerlimit of the tensile strength may be set to 580 MPa, 600 MPa, or 620MPa. The upper limit of the tensile strength may be set to 780 MPa, 760MPa, or 750 MPa. In the present embodiment, the room temperature is setto 20° C. in principle.

Next, a method of manufacturing the steel for low temperature accordingto the present embodiment will be described. If the Ni steel accordingto the present embodiment has the above-described configurationregardless of the manufacturing method, the effect can be obtained.However, for example, according to the following manufacturing method,the Ni steel according to the present embodiment can be obtained stably.

The method of manufacturing the Ni steel according to the presentembodiment includes: a step of adjusting the amounts of elements in astate in which the temperature of molten steel is set to 1650° C. orlower, the concentration of O in the molten steel is set to 0.01% orless, and the concentration of S in the molten steel is set to 0.02% orless, and thereafter manufacturing a steel piece by continuous casting;a step of heating the obtained steel piece to 950° C. to 1160° C. andretaining the steel piece for 30 minutes to 180 minutes; a step ofperforming hot rolling on the steel piece under the condition that acumulative rolling reduction at 950° C. or lower is 80% or more and 95%or less and a finishing temperature is 650° C. to 850° C.; a step ofperforming water cooling on the hot-rolled steel plate to roomtemperature with a cooling start temperature of 550° C. to 850° C. orperforming air cooling on the hot-rolled steel plate; a step ofperforming reheating hardening on the hot-rolled steel plate at areheating hardening temperature of 720° C. to 880° C. for a retentiontime of 20 minutes to 180 minutes with a water cooling finishingtemperature of 200° C. or lower; a step of performing an intermediateheat treatment on the hot-rolled steel plate at an intermediate heattreatment temperature of 610° C. to 650° C. for a retention time of 20minutes to 180 minutes; and a step of performing tempering on thehot-rolled steel plate at a tempering temperature of 530° C. to 570° C.for a retention time of 20 minutes to 180 minutes. These manufacturingconditions are preferably further limited according to the Ni contentand the like.

Hereinafter, details of the manufacturing conditions will be described.

Before the hot rolling, a slab is heated. Here, the heating temperatureis set to 950° C. to 1160° C. When the heating temperature of the slabis lower than 950° C., there are cases where the heating temperature islower than a predetermined hot rolling finishing temperature. When theheating temperature of the slab exceeds 1160° C., austenite grain sizesbecome coarse during the heating, and the toughness of the Ni steel atan extremely low temperature may decrease. Due to solutionizing of AlN,the retention time of the heating is 30 minutes to 180 minutes. In acase where the Ni content is small, the heating temperature of the hotrolling is set to 950° C. to 1100° C. Even in a case where the Nicontent is small, the retention time of the heating is 30 minutes to 180minutes.

In the hot rolling, when the cumulative rolling reduction at 950° C. orlower is less than 80%, refinement of austenite grains byrecrystallization of austenite in the slab during the rolling isinsufficient, and there are cases where a portion of the austenitegrains after the rolling is coarsened and the extremely low temperaturetoughness of the Ni steel decreases. Therefore, the lower limit of thecumulative rolling reduction at 950° C. or lower is set to 80%.Homogenous refinement of prior austenite grains by recrystallizationduring rolling is particularly important in securing the extremely lowtemperature toughness of the Ni steel according to the presentembodiment, and strict restriction on the rolling temperature and thecumulative rolling reduction is required. On the other hand, when thecumulative rolling reduction at 950° C. or lower exceeds 95%, therolling time becomes long and problems occur in productivity in somecases, so that the upper limit of the cumulative rolling reduction at950° C. or lower is set to 95%.

When the finishing temperature of the hot rolling is lower than 650° C.,deformation resistance increases and the load on a rolling millincreases. Therefore, the lower limit of the finishing temperature ofthe hot rolling is set to 650° C. When the finishing temperature of thehot rolling exceeds 850° C., dislocations introduced by rolling arereduced due to recovery and there are cases where the extremely lowtemperature toughness of the Ni steel is insufficient, or there arecases where the yield stress of the Ni steel at room temperature isinsufficient. Therefore, the upper limit of the finishing temperature isset to 850° C. In a case where the Ni content is small, the upper limitof the finishing temperature is set to 800° C.

Although cooling performed after the hot rolling may be either watercooling or air cooling, it is preferable to perform water cooling. Inthe case of water cooling, it is preferable that the water coolingfinishing temperature is set to 200° C. or lower, and the water coolingstart temperature is set to 550° C. to 850° C.

The reheating hardening is a heat treatment in which the hot-rolledsteel plate is heated to a reheating hardening temperature, retained atthe reheating hardening temperature, and then cooled, and is effectivefor refinement of prior austenite. The reheating hardening temperature(heating temperature during reheating) is set to 720° C. to 880° C. Whenthe reheating hardening temperature is lower than 720° C., a portionthat does not transform into austenite remains in the hot-rolled steelplate after being subjected to the reheating hardening, and there arecases where the yield stress or tensile strength of the Ni steel at roomtemperature decreases. When the reheating hardening temperature exceeds880° C., the austenite grain size becomes coarse and AlN becomes coarse,so that the extremely low temperature toughness of the Ni steeldecrease. The retention time during the reheating hardening is set to 20minutes to 180 minutes. When the retention time of the reheatinghardening is shorter than 20 minutes, there is concern that austenitictransformation may not proceed sufficiently. On the other hand, in acase where the retention time of the reheating hardening is longer than180 minutes, there is concern that austenite grains may become coarse.As cooling during the reheating hardening, tempering to 200° C. or lowerfrom the heating temperature during the reheating is performed. In acase where the Ni content is small, the reheating hardening temperatureis set to 740° C. to 780° C. The average cooling rate during thehardening is set to 10° C./s or more.

The intermediate heat treatment is a heat treatment in which thehot-rolled steel plate after being subjected to the reheating hardeningis heated to the intermediate heat treatment temperature, retained atthe intermediate heat treatment temperature, and then cooled, and iseffective for refinement of the effective grain size, which contributesto the improvement of the extremely low temperature toughness andsecuring of the austenite phase. The intermediate heat treatmenttemperature is set to 610° C. to 650° C. When the intermediate heattreatment temperature is lower than 610° C., austenitic transformationbecomes insufficient, and the fraction of tempered martensite which isexcessively tempered increases, so that there are cases where thestrength of the Ni steel decreases. Furthermore, when the intermediateheat treatment temperature is lower than 610° C., there are cases wherethe extremely low temperature toughness of the Ni steel decreases.

When the intermediate heat treatment temperature exceeds 650° C., theaustenitic transformation in the hot-rolled steel plate proceedsexcessively. As a result, stabilization of austenite is insufficient,and an austenite phase having a volume fraction of 2.0 vol % or more maynot be secured in the thickness middle portion of the Ni steel or theextremely low temperature toughness of the Ni steel may decrease. Theretention time of the intermediate heat treatment is set to 20 minutesto 180 minutes. In a case where the retention time of the intermediateheat treatment is shorter than 20 minutes, there is concern that theaustenitic transformation does not proceed sufficiently. On the otherhand, in a case where the retention time of the intermediate heattreatment is longer than 180 minutes, there is concern that carbidesadversely affecting the toughness may be precipitated. A cooling methodduring the intermediate heat treatment is water cooling in order toavoid tempering embrittlement, and water cooling to 200° C. or lower isperformed. The average cooling rate during the water cooling is set to8° C./s or more.

The tempering is a heat treatment in which the hot-rolled steel plateafter being subjected to the intermediate heat treatment is heated tothe tempering temperature, retained at the tempering temperature, andthen cooled, and is effective for securing the austenite phase. Thetempering temperature is set to 530° C. to 570° C. When the temperingtemperature is lower than 530° C., the austenite phase in the thicknessmiddle portion of the Ni steel cannot be secured in a volume fraction of2.0 vol % or more, and there are cases where the extremely lowtemperature toughness of the Ni steel is insufficient. When thetempering temperature exceeds 570° C., there are cases where the amountof the austenite phase of the Ni steel at room temperature exceeds 20.0vol % in terms of volume fraction. When this Ni steel is cooled to anextremely low temperature, a portion of austenite is transformed intohigh C martensite, and there are cases where the extremely lowtemperature toughness of the Ni steel decreases. For this reason, theupper limit of the tempering temperature is 570° C. or lower. Theretention time of the tempering is set to 20 minutes to 180 minutes. Ina case where the retention time of the tempering is shorter than 20minutes, there is concern that the stability of austenite may not besufficiently secured. On the other hand, in a case where the retentiontime of the tempering is longer than 180 minutes, there is concern thatcarbides adversely affecting the toughness may be precipitated, and thetensile strength may be insufficient. A cooling method during thetempering is water cooling in order to avoid tempering embrittlement,and water cooling to 200° C. or lower is performed. The average coolingrate during the water cooling is set to 5° C./s or more.

EXAMPLES

Examples of the present invention will be described below. However, thefollowing examples are examples of the present invention, and thepresent invention is not limited to the examples described below.

Example 1: Ni Steel Having Ni Content of 11.5% or More

Steel was melted by a converter and slabs having a thickness of 150 mmto 300 mm were manufactured by continuous casting. Tables 1 and 2 showthe chemical compositions of Kinds of steel A1 to A24. These slabs wereheated, subjected to controlled rolling, directly subjected to watercooling or air cooling, and subjected to heat treatments includingreheating hardening, an intermediate heat treatment, and tempering,whereby steel plates were manufactured. The retention time of theheating of the hot rolling was set to 30 minutes to 120 minutes, and theretention time of the heat treatments including the reheating hardening,the intermediate heat treatment, and the tempering was set to 20 minutesto 60 minutes. In a case of performing water cooling after the hotrolling, water cooling to 200° C. or lower was performed. Means forcooling in the heat treatments including the reheating hardening, theintermediate heat treatment, and the tempering was water cooling, andwater cooling to 200° C. or lower from the treatment temperature of eachof the heat treatments was performed. Samples were taken from the steelplates, and the metallographic structure, tensile properties, andtoughness thereof were evaluated.

TABLE 1 Steel Chemical composition (mass %, remainder consists of Fe andimpurities) material C Si Mn P S Cu Ni Cr Mo Al Nb Ti A1 0.030 0.19 0.250.0060 0.0010 0.05 11.8 0.03 0.03 0.025 A2 0.070 0.08 0.31 0.0040 0.002211.9 0.024 A3 0.033 0.30 0.36 0.0030 0.0009 12.3 0.02 0.028 0.013 A40.038 0.08 0.20 0.0030 0.0007 11.5 0.025 A5 0.032 0.25 0.80 0.00400.0008 0.04 12.2 0.017 0.006 A6 0.035 0.03 0.22 0.0020 0.0040 12.0 0.500.40 0.025 A7 0.063 0.11 0.45 0.0030 0.0011 12.0 0.11 0.20 0.060 A80.058 0.20 0.35 0.0080 0.0013 12.1 0.020 0.020 0.020 A9 0.067 0.05 0.750.0060 0.0013 0.50 12.4 0.010 A10 0.031 0.27 0.21 0.0030 0.0005 0.0211.6 0.35 0.015 0.015 A11 0.052 0.07 0.60 0.0030 0.0031 11.7 0.32 0.150.040 0.008 A12 0.050 0.12 0.41 0.0040 0.0007 0.24 11.8 0.050 A13 0.0460.14 0.40 0.0050 0.0007 0.43 11.7 0.033 Steel Chemical composition (mass%, remainder consists of Fe and impurities) material V B Ca REM N O NoteA1 0.0025 0.0009 Present A2 0.0025 0.0015 Invention A3 0.061 0.00100.0026 0.0013 Example A4 0.020 0.0025 0.0012 A5 0.0005 0.0015 0.00350.0016 A6 0.0008 0.0040 0.0012 A7 0.0020 0.0053 0.0008 A8 0.080 0.00280.0018 0.0027 A9 0.0020 0.0027 0.0011 A10 0.0040 0.0040 0.0013 A11 0.0450.0050 0.0015 0.0016 A12 0.0060 0.0007 A13 0.0020 0.0030 Blank meansthat no element is intentionally added.

TABLE 2 Steel Chemical composition (mass %, remainder consists of Fe andimpurities) material C Si Mn P S Cu Ni Cr Mo Al Nb Ti A14 0.026 0.240.24 0.0030 0.0015 12.0 0.030 A15 0.077 0.26 0.26 0.0030 0.0015 12.00.030 A16 0.055 0.34 0.26 0.0030 0.0014 12.1 0.032 A17 0.054 0.27 0.150.0030 0.0010 12.0 0.031 A18 0.048 0.21 0.89 0.0030 0.0005 12.0 0.030A19 0.049 0.22 0.21 0.0090 0.0032 0.10 12.2 0.15 0.031 A20 0.060 0.220.26 0.0050 0.0046 11.9 0.13 0.08 0.020 A21 0.063 0.21 0.72 0.00500.0009 12.0 0.61 0.020 A22 0.064 0.05 0.72 0.0050 0.0012 0.10 12.0 0.066A23 0.036 0.05 0.71 0.0030 0.0006 12.0 0.020 0.024 A24 0.035 0.05 0.710.0030 0.0008 11.7 0.020 0.026 Steel Chemical composition (mass %,remainder consists of Fe and impurities) material V B Ca REM N O NoteA14 0.0030 0.0014 Comparative A15 0.0030 0.0014 Example A16 0.00300.0014 A17 0.003 0.0029 0.0013 A18 0.0003 0.0023 0.0031 0.0015 A190.0045 0.0015 A20 0.0031 0.0010 A21 0.0035 0.0020 0.0015 A22 0.00250.0013 A23 0.0036 0.0030 0.0015 A24 0.0050 0.0074 0.0012 Blank meansthat no element is intentionally added. Underline means outside therange of the present invention.

The average grain size of prior austenite grains (the average grain sizeof prior γ) to be measured in a section of a thickness middle portionparallel to a rolling direction and a thickness direction was measuredin a section (L-section) of a thickness middle portion parallel to arolling direction and a thickness direction as an observed section. Theaverage grain size of the prior austenite grains was measured accordingto JIS G 0551. First, the observed section of the sample was corrodedwith a saturated aqueous solution of picric acid to reveal the prioraustenite grain boundaries, and thereafter five or more visual fieldswere photographed with a scanning electron microscope at a magnificationof 1,000-fold or 2,000-fold. After identifying the prior austenite grainboundaries using the structural photographs, the circle equivalent grainsizes (diameters) of at least 20 prior austenite grains were obtained byimage processing, and the average value thereof was determined as theaverage grain size of the prior austenite grains.

In addition, in the steel of the present invention, the refinement ofthe prior austenite grain size, suppression of the P content, and thelike are carried out so that fracture is less likely to occur at theprior austenite grain boundaries. Therefore, it may be difficult toidentify the prior austenite grain boundaries by corrosion. In such acase, after heating the sample to 450° C. to 490° C., a heat treatmentof temperature retention for one hour or longer was performed, and thenthe average grain size of the prior austenite grains was measured by themethod described above.

In a case where identification of the prior austenite grain boundarieswas difficult even if the heat treatment at 450° C. to 490° C. wasperformed, a Charpy test piece was taken from the heat-treated sample,and the sample subjected to an impact test at −196° C. and fractured atthe prior austenite grain boundaries was used. In this case, a crosssection of a fracture surface at the section (L-section) parallel to therolling direction and the thickness direction was created and corroded,and thereafter, the prior austenite grain sizes were measured byidentifying the prior austenite grain boundaries of the cross section ofthe fracture surface of the thickness middle portion with the scanningelectron microscope. When the prior austenite grain boundaries areembrittled by a heat treatment, minute cracks are generated at the prioraustenite grain boundaries due to an impact load during the Charpy test,so that the prior austenite grain boundaries are easily identified.

The average aspect ratio of the prior austenite grains (average aspectratio of prior γ grains) measured in the section of the thickness middleportion parallel to the rolling direction and the thickness directionwas obtained as a ratio between the maximum value (length in the rollingdirection) and the minimum value (thickness in the thickness direction)of the length of a region (prior austenite grain) surrounded by theprior austenite grain boundary identified as described above. Theaverage aspect ratios of at least 20 prior austenite grains weremeasured, and the average value thereof was calculated to obtain theaverage aspect ratio.

The volume fraction of the austenite phase (volume fraction of γ phase)contained in the metallographic structure of the thickness middleportion was measured by taking a sample parallel to the plate surfacefrom the thickness middle portion and performing an X-ray diffractionmethod thereon. The volume fraction of the austenite phase wasdetermined from the ratio between the integrated intensities ofaustenite (face-centered cubic structure) and tempered martensite(body-centered cubic structure) of X-ray peaks.

The average effective grain size measured in the section of thethickness middle portion parallel to the rolling direction and thethickness direction was measured by using an EBSD analyzer attached tothe scanning electron microscope, with the section (L-section) of thethickness middle portion parallel to the rolling direction and thethickness direction as an observed section. Observation of five or morevisual fields was performed at a magnification of 2,000-fold, a boundaryof a metallographic structure having an orientation difference of 15° ormore was regarded as a grain boundary, grains surrounded by the grainboundaries were regarded as effective grains, the circle equivalentgrain sizes (diameters) were obtained from the areas by imageprocessing, and the average value of the circle equivalent grain sizeswere determined as an average effective grain size.

By taking a 1A full-thickness tensile test piece specified in JIS Z 2241whose longitudinal direction was parallel to the rolling direction (Ldirection), strength (yield stress and tensile strength) was measured atroom temperature by the method specified in JIS Z 2241. The target valueof the yield stress is 460 MPa to 580 MPa, and the target value of thetensile strength is 560 MPa to 680 MPa. The yield stress was a loweryield stress. However, there were many cases where no clear lower yieldstress was observed, and in that case, the 0.2% proof stress was taken.

Regarding the extremely low temperature toughness, a CT test piece offull thickness with front and rear surfaces each ground 0.5 mm was takenin a direction (C direction) perpendicular to the rolling direction. AJ-R curve was created according to the unloading compliance methodspecified in ASTM standard E1820-13 in liquid hydrogen (−253° C.), and aJ value was converted into a Kw value. The target value of the extremelylow temperature toughness is 150 MPa·√m or more.

Tables 3 and 4 show the plate thickness, manufacturing method, basemetal properties, and metallographic structure of steel materials (Steelmaterials Nos. 1 to 32) manufactured using slabs having the chemicalcompositions of Kinds of steel A1 to A24.

TABLE 3 Heating, rolling, and heat treatment conditions CumulativeInter- Heating rolling Water mediate temperature reduction Rollingcooling start Reheating heat Plate during at 950° C. finishingtemperature hardening treatment Tempering Manufacturing Steel thicknessrolling or lower temperature after rolling temperature temperaturetemperature condition No. material (mm) (° C.) (%) (° C.) (° C.) (° C.)(° C.) (° C.) a1 A1 12 1160 95 800 750 850 630 560 a2 A2 20 1100 80 830Air cooling 830 640 550 after rolling a3 A3 20 970 92 720 680 760 630530 a4 A4 12 980 95 790 Air cooling 720 620 530 after rolling a5 A5 301030 80 770 730 780 640 550 a6 A6 30 1000 91 800 Air cooling 770 620 550after rolling a7 A7 30 1160 88 800 810 880 640 560 a8 A8 30 970 90 700660 760 620 540 a9 A9 30 1070 85 840 810 820 640 540 a10 A9 30 1070 85830 800 820 640 540 a11 A9 30 1070 85 830 800 820 640 540 a12 A9 30 108085 830 810 820 640 540 a13 A10 30 1050 88 790 760 800 640 570 a14 A11 301060 88 840 Air cooling 760 650 540 after rolling a15 A12 30 960 90 670630 750 630 530 a16 A13 30 950 90 650 610 750 630 540 Metallographicstructure Average Base metal properties grain Average Volume AverageExtremely size of aspect fraction effective low prior γ ratio of of γgrain Yield Tensile temperature Manufacturing Steel grains prior γ phasesize stress strength toughness* condition No. material (μm) grains (%)(μm) (MPa) (MPa) (MPa · √m) Note a1 A1 11.0 1.7 12.0 4.5 558 623 156Present a2 A2 9.6 1.6 9.1 7.0 499 581 152 Invention a3 A3 4.3 2.1 7.52.8 527 598 165 Example a4 A4 5.2 1.8 8.5 3.2 534 624 163 a5 A5 6.8 1.96.3 3.0 483 570 164 a6 A6 6.3 1.7 6.8 4.7 555 663 155 a7 A7 15.0 1.014.0 8.6 573 673 150 a8 A8 3.5 2.2 2.4 2.6 525 613 189 a9 A9 8.2 1.4 7.63.8 546 645 160 a10 A9 8.3 1.4 2.0 3.7 569 665 151 a11 A9 10.5 1.2 2.15.6 534 641 152 a12 A9 8.2 1.4 15.0 3.8 555 645 152 a13 A10 7.5 1.8 8.03.5 510 602 162 a14 A11 9.3 1.5 6.7 5.1 485 580 154 a15 A12 3.4 2.3 4.52.3 495 580 195 a16 A13 3.3 2.4 5.6 2.0 480 567 210 *Extremely lowtemperature toughness is the K_(I)C value (converted from J value) inliquid hydrogen (−253° C.), the unit is MPa · √m.

TABLE 4 Heating, rolling, and heat treatment conditions Cumulative WaterInter- Heating rolling cooling mediate temperature reduction Rollingstart Reheating heat Plate during at 950° C. finishing temperaturehardening treatment Tempering Manufacturing Steel thickness rolling orlower temperature after rolling temperature temperature temperaturecondition No. material (mm) (° C.) (%) (° C.) (° C.) (° C.) (° C.) (°C.) a17 A14 30 1100 80 800 770 800 640 560 a18 A15 30 1080 85 780 750760 620 550 a19 A16 30 1060 90 770 740 770 650 540 a20 A17 30 1050 90760 720 750 630 550 a21 A18 30 1050 85 800 760 830 650 540 a22 A19 301080 85 780 750 760 630 550 a23 A20 30 1070 80 800 750 800 630 550 a24A21 30 1070 80 790 760 780 640 560 a25 A22 30 1070 80 770 740 780 640530 a26 A23 30 1070 80 780 740 820 630 530 a27 A24 30 1080 80 780 750830 630 540 a28 A8 30 930 85 640 610 860 630 550 a29 A7 30 1180 88 830800 860 640 550 a30 A7 30 1160 50 840 810 850 640 550 a31 A7 30 1130 88880 850 840 650 560 a32 A9 30 1080 85 840 800 910 640 530 Metallographicstructure Average Base metal properties grain Average Volume AverageExtremely size of aspect fraction effective low prior γ ratio of of γgrain Yield Tensile temperature Manufacturing Steel grains prior γ phasesize stress strength toughness* condition No. material (μm) grains (%)(μm) (MPa) (MPa) (MPa · √m) Note a17 A14 7.3 1.2 1.2 4.0 453 552 66Comparative a18 A15 6.6 1.4 5.5 3.5 506 602 67 Example a19 A16 6.8 1.411.4  3.6 486 578 58 a20 A17 6.1 1.5 0.8 3.3 455 563 75 a21 A18 7.6 1.110.5  4.0 545 647 64 a22 A19 6.5 1.2 5.2 3.4 496 590 80 a23 A20 7.2 1.14.3 3.9 526 618 65 a24 A21 7.1 1.3 14.0  3.5 560 657 62 a25 A22 7.0 1.412.5  3.4 543 639 59 a26 A23 7.4 2.5 4.5 4.1 503 594 67 a27 A24 7.5 1.03.6 4.2 499 585 70 a28 A8 3.6 2.5 7.2 2.8 713 817 69 a29 A7 15.7  1.07.2 8.7 570 667 125 a30 A7 15.6  1.1 7.0 8.8 565 661 120 a31 A7 16.2 1.0 7.2 9.1 564 664 118 a32 A9 16.3  1.2 5.6 9.3 571 678 123 *Extremelylow temperature toughness is the K_(I)C value (converted from J value)in liquid hydrogen (−253° C.). the unit is MPa · √m.

As is apparent from Tables 3 and 4, in Manufacturing Nos. a1 to a16, thetensile strength and the yield stress at room temperature, and theextremely low temperature toughness satisfied the target values.

On the other hand, in Manufacturing No. a17, the C content was small,and in Manufacturing No. a20, the Mn content was small, so that thetensile strength was low and the extremely low temperature toughness hadalso decreased. In each of Manufacturing Nos. a18, a19, and a21 to a25,the C content, Si content, Mn content, P content, S content, Cr content,and Al content were large, and the extremely low temperature toughnesshad decreased. In Manufacturing No. a26, the Nb content and the Bcontent were too large, and the extremely low temperature toughness haddecreased. In Steel material No. a27, the Ti content and the N contentwere large, and the extremely low temperature toughness had decreased.

Manufacturing Nos. a28 to a32 are examples in which manufacturingconditions that deviated from preferable ranges are adopted. InManufacturing No. a28, the heating temperature during rolling was lowand the rolling finishing temperature was also low, so that the tensilestrength had increased excessively and the extremely low temperaturetoughness had decreased. In each of Manufacturing Nos. a29, a31, anda32, the heating temperature during rolling, the rolling finishingtemperature, and the reheating hardening temperature were high, and theprior austenite grain size had increased, so that the extremely lowtemperature toughness had decreased. In Manufacturing No. a30, thecumulative rolling reduction at 950° C. or lower was small, and theprior austenite grain size had increased, so that the extremely lowtemperature toughness had decreased.

Example 2: Ni Steel Having Ni Content of Less than 11.5%

Steel was melted by a converter and slabs having a thickness of 100 mmto 300 mm were manufactured by continuous casting. Tables 5 and 6 showthe chemical compositions of Kinds of steel B1 to B24. These slabs wereheated, subjected to controlled rolling, directly subjected to watercooling, and subjected to heat treatments including reheating hardening,an intermediate heat treatment, and tempering, whereby steel plates weremanufactured. The retention time of the heating of the hot rolling wasset to 30 minutes to 120 minutes, and the retention time of the heattreatments including the reheating hardening, the intermediate heattreatment, and the tempering was set to 20 minutes to 60 minutes. Watercooling to 200° C. or lower was performed after the hot rolling. Meansfor cooling in the heat treatments including the reheating hardening,the intermediate heat treatment, and the tempering was water cooling,and water cooling to 200° C. or lower from the treatment temperature ofeach of the heat treatments was performed. Samples were taken from thesteel plates, and the metallographic structure, tensile properties, andtoughness thereof were evaluated.

TABLE 5 Steel Chemical composition (mass %, remainder consists of Fe andimpurities) material C Si Mn P S Cu Ni Cr Mo Al Nb Ti B1 0.030 0.18 0.400.0040 0.0015 10.7 0.024 0.007 B2 0.060 0.06 0.44 0.0030 0.0017 10.80.03 0.01 0.023 B3 0.032 0.19 0.33 0.0020 0.0010 11.0 0.02 0.04 0.011 B40.035 0.10 0.30 0.0050 0.0009 0.05 11.4 0.020 0.001 B5 0.042 0.19 0.500.0040 0.0007 0.02 11.3 0.04 0.019 B6 0.036 0.14 0.41 0.0040 0.0030 11.20.35 0.40 0.017 0.009 B7 0.050 0.10 0.42 0.0030 0.0015 11.2 0.15 0.200.050 0.009 B8 0.056 0.19 0.38 0.0060 0.0012 11.3 0.05 0.025 0.015 0.015B9 0.045 0.16 0.40 0.0050 0.0010 0.50 10.5 0.06 0.010 B10 0.035 0.170.41 0.0040 0.0005 10.6 0.038 0.002 B11 0.049 0.06 0.39 0.0030 0.00250.03 10.9 0.03 0.045 B12 0.048 0.08 0.43 0.0030 0.0006 0.20 11.1 0.040.040 0.012 B13 0.045 0.16 0.44 0.0050 0.0007 0.40 11.0 0.25 0.016 0.0100.005 Steel Chemical composition (mass %, remainder consists of Fe andimpurities) material V B Ca REM N O Note B1 0.0002 0.0020 0.0010 PresentB2 0.030 0.0021 0.0007 Invention B3 0.0002 0.0017 0.0024 0.0012 ExampleB4 0.012 0.0030 0.0014 B5 0.0002 0.0025 0.0012 B6 0.0025 0.0016 B7 0.0410.0010 0.0005 0.0030 0.0027 0.0009 B8 0.060 0.0016 0.0018 0.0016 B90.0020 0.0025 0.0032 0.0020 B10 0.0040 0.0044 0.0011 B11 0.0002 0.00500.0015 0.0015 B12 0.0050 0.0010 B13 0.010 0.0026 0.0030 Blank means thatno element is intentionally added.

TABLE 6 Steel Chemical composition (mass %, remainder consists of Fe andimpurities) material C Si Mn P S Cu Ni Cr Mo Al Nb Ti B14 0.024 0.180.48 0.0040 0.0025 11.2 0.14 0.020 0.012 0.012 B15 0.070 0.18 0.450.0050 0.0016 11.3 0.030 B16 0.055 0.21 0.44 0.0050 0.0010 11.1 0.015B17 0.033 0.17 0.20 0.0050 0.0014 10.6 0.030 0.010 0.010 B18 0.052 0.170.63 0.0060 0.0025 0.02 11.2 0.20 0.043 0.012 B19 0.055 0.16 0.49 0.00700.0020 11.2 0.30 0.038 0.009 B20 0.056 0.16 0.48 0.0030 0.0044 10.8 0.280.040 0.010 B21 0.054 0.05 0.38 0.0050 0.0024 11.0 0.43 0.015 0.0120.010 B22 0.058 0.08 0.48 0.0050 0.0020 0.40 10.8 0.06 0.15 0.060 B230.056 0.18 0.48 0.0050 0.0027 0.25 11.2 0.30 0.15 0.040 0.019 B24 0.0550.19 0.47 0.0050 0.0027 0.15 10.8 0.14 0.15 0.013 0.018 Steel Chemicalcomposition (mass %, remainder consists of Fe and impurities) material VB Ca REM N O Note B14 0.045 0.0002 0.0032 0.0012 Comparative B15 0.00300.0014 Example B16 0.0025 0.0034 0.0020 B17 0.035 0.0033 0.0016 B180.0002 0.0036 0.0014 B19 0.0039 0.0019 B20 0.035 0.0012 0.0037 0.0015B21 0.044 0.0015 0.0047 0.0021 B22 0.0048 0.0020 B23 0.0032 0.00460.0016 B24 0.0002 0.0032 0.0060 0.0025 Blank means that no element isintentionally added. Underline means outside the range of the presentinvention.

A method of identifying the metallographic structure of the sample, amethod of evaluating the mechanical properties, and acceptance criteriafor the mechanical properties were the same as those for the samplesdisclosed in Tables 1 to 4. Tables 3 and 4 show the plate thickness,manufacturing method, base metal properties, and metallographicstructure of steel materials (Manufacturing Nos. b1 to b30) manufacturedusing slabs having the chemical compositions of Kinds of steel B1 toB24.

TABLE 7 Heating, rolling, and heat treatment conditions Cumulative WaterInter- Heating rolling cooling mediate temperature reduction Rollingstart Reheating heat Plate during at 950° C. finishing temperaturehardening treatment Tempering Manufacturing Steel thickness rolling orlower temperature after rolling temperature temperature temperaturecondition No. material (mm) (° C.) (%) (° C.) (° C.) (° C.) (° C.) (°C.) b1 B1 12 1050 95 800 710 760 650 560 b2 B2 18 1070 82 780 680 750630 540 b3 B3 14 1050 90 800 710 770 640 550 b4 B4 12 1060 88 770 680740 640 530 b5 B5 18 1030 90 790 720 760 630 530 b6 B6 20 1050 83 650550 770 630 540 b7 B7 20 1100 80 680 590 780 630 540 b8 B8 20 990 90 770690 760 630 540 b9 B8 20 1030 83 780 710 760 640 540 b10 B9 18 950 82780 740 760 630 540 b11 B10 14 970 92 760 700 780 640 540 b12 B11 161020 87 770 680 750 630 530 b13 B12 18 980 85 720 630 750 620 530 b14B13 20 1000 87 700 590 750 630 540 Metallographic structure Average Basemetal properties grain Average Volume Average Extremely size of aspectfraction effective low prior γ ratio of of γ grain Yield Tensiletemperature Manufacturing Steel grains prior γ phase size stressstrength toughness* condition No. material (μm) grains (%) (μm) (MPa)(MPa) (MPa · √m) Note b1 B1 8.0 1.1 2.4 5.0 473 568 220 Present b2 B26.8 1.2 4.9 2.5 524 633 179 Invention b3 B3 7.7 1.0 3.5 3.6 486 596 196Example b4 B4 7.2 1.5 2.3 4.9 460 560 215 b5 B5 7.8 1.1 4.0 3.4 521 635183 b6 B6 3.0 2.4 3.6 2.3 580 675 150 b7 B7 4.2 1.7 4.6 2.4 566 670 155b8 B8 6.7 1.9 4.7 3.0 550 659 157 b9 B8 7.5 1.6 2.0 5.5 486 586 151 b10B9 7.1 1.3 4.0 3.3 507 615 185 b11 B10 4.8 1.7 2.5 4.7 478 577 212 b12B11 6.7 1.4 4.8 4.4 485 580 204 b13 B12 5.5 2.2 5.0 4.0 537 634 170 b14B13 4.3 2.3 4.2 2.0 564 650 166 *Extremely low temperature toughness isthe K_(I)C value (converted from J value) in liquid hydrogen (−253° C.),the unit is MPa · √m.

TABLE 8 Heating, rolling, and heat treatment conditions Cumulative WaterInter- Heating rolling cooling mediate temperature reduction Rollingstart Reheating heat Plate during at 950° C. finishing temperaturehardening treatment Tempering Manufacturing Steel thickness rolling orlower temperature after rolling temperature temperature temperaturecondition No. material (mm) (° C.) (%) (° C.) (° C.) (° C.) (° C.) (°C.) b15 B14 20 1040 83 790 720 780 640 540 b16 B15 20 1050 83 780 710750 630 540 b17 B16 20 1080 83 800 740 770 640 550 b18 B17 20 1080 83790 720 780 650 560 b19 B18 20 1100 83 800 740 770 630 550 b20 B19 201050 83 780 710 770 630 550 b21 B20 20 1080 83 790 700 770 640 550 b22B21 20 1070 83 790 730 760 630 550 b23 B22 20 1080 83 770 710 770 630550 b24 B23 20 1050 83 760 690 770 630 550 b25 B24 20 1060 83 770 690770 640 550 b26 B12 18 930 85 630 600 750 620 530 b27 B1 12 1130 90 800750 760 650 560 b28 B5 20 1090 75 790 730 760 630 530 b29 B6 20 1080 83830 770 770 630 540 b30 B7 20 1080 80 700 630 800 630 540 Metallographicstructure Average Base metal properties grain Average Volume AverageExtremely size of aspect fraction effective low prior γ ratio of of γgrain Yield Tensile temperature Manufacturing Steel grains prior γ phasesize stress strength toughness* condition No. material (pm) grains (%)(μm) (MPa) (MPa) (MPa · √m) Note b15 B14 7.2 1.5 1.2 4.7 470 564 98Comparative b16 B15 6.4 1.2 4.6 4.3 480 573 116 Example b17 B16 7.1 1.03.5 4.6 470 566 113 b18 B17 6.5 1.1 1.6 4.8 465 563 115 b19 B18 6.8 1.54.7 3.2 526 627 120 b20 B19 6.7 1.4 3.4 3.4 522 625 112 b21 B20 6.8 1.23.2 3.4 530 638 105 b22 B21 6.9 1.6 4.2 3.3 532 643 124 b23 B22 4.0 1.33.6 3.1 550 650 130 b24 B23 6.7 2.5 3.4 2.3 570 674 116 b25 B24 3.7 1.23.5 2.8 560 667 82 b26 B12 3.4 2.5 4.3 2.1 590 695 80 b27 B1 9.2 1.0 2.65.6 485 578 105 b28 B5 8.6 1.1 4.1 5.0 535 640 124 b29 B6 8.5 1.4 3.95.5 580 678 115 b30 B7 8.7 1.5 4.5 5.3 570 675 120 Underline meansoutside the range of the present invention. *Extremely low temperaturetoughness is the K_(I)C value (converted from J value) in liquidhydrogen (−253° C.), the unit is MPa · √m.

As is apparent from Tables 7 and 8, in Manufacturing Nos. b1 to b14, thetensile strength and the yield stress at room temperature, and theextremely low temperature toughness satisfied the target values.

On the other hand, in Manufacturing No. b15, the C content was small,and in Manufacturing No. b18, the Mn content was small, so that theextremely low temperature toughness had decreased. In each ofManufacturing Nos. b16, b17, and b19 to b23, the C content, Si content,Mn content, P content, S content, Cr content, and Al content were large,and the extremely low temperature toughness had decreased. InManufacturing No. b24, the Nb content and the B content were too large,and the extremely low temperature toughness had decreased. InManufacturing No. b25, the Ti content and the N content were large, andthe extremely low temperature toughness had decreased.

Manufacturing Nos. b26 to b30 are examples in which manufacturingconditions that deviated from preferable ranges are adopted. InManufacturing No. b26, the heating temperature during rolling was lowand the rolling finishing temperature was also low, so that the tensilestrength had increased excessively and the extremely low temperaturetoughness had decreased. In each of Manufacturing Nos. b27, b29, andb30, the heating temperature during rolling, the rolling finishingtemperature, and the reheating hardening temperature were high, theprior austenite grain size had increased, and the effective grain sizehad also increased, so that the extremely low temperature toughness haddecreased. In Manufacturing No. b28, the cumulative rolling reduction at950° C. or lower was small, and the prior austenite grain size hadincreased, so that the extremely low temperature toughness haddecreased.

1. A nickel-containing steel comprising, as a chemical composition, bymass %: C: 0.030% to 0.070%; Si: 0.03% to 0.30%; Mn: 0.20% to 0.80%; Ni:10.5% to 12.4%; Al: 0.010% to 0.060%; N: 0.0015% to 0.0060%; O: 0.0007%to 0.0030%; Cu: 0% to 0.50%; Cr: 0% to 0.50%; Mo: 0% to 0.40%; Nb: 0% to0.020%; V: 0% to 0.080%; Ti: 0% to 0.020%; B: 0% to 0.0020%; Ca: 0% to0.0040%; REM: 0% to 0.0050%; P: 0.0080% or less; S: 0.0040% or less; anda remainder: Fe and impurities, wherein a metallographic structure of athickness middle plane contains 2.0 vol % to 20.0 vol % of an austenitephase, an average grain size of prior austenite grains, measured inaccordance with JIS G 0551, measured in a section of the thickness planeparallel to a rolling direction and a thickness direction is 3.0 μm to15.0 μm, an average aspect ratio of the prior austenite grains, whereinthe aspect ratio of the prior austenite grains is defined as: length ofthe prior austenite grains in a rolling direction/thickness of the prioraustenite grains in the thickness direction, measured in the section ofthe thickness middle plane parallel to the rolling direction and thethickness direction is 1.0 to 2.4, a plate thickness is 4.5 mm to 20 mm,when the nickel-containing steel contains Ni: less than 11.5%, thenickel-containing steel contains C: 0.060% or less, Si: 0.19% or less,Mn: 0.30% to 0.50%, A1: 0.050% or less, N: 0.0050% or less, Cr: 0.35% orless, Nb: 0.015% or less, V: 0.060% or less, Ti: 0.015% or less, P:0.0060% or less, and S: 0.0030% or less, and the average grain size ofthe prior austenite grains, measured in accordance with JIS G 0551, is8.0 μm or less, a yield stress at room temperature, measured inaccordance with JIS Z 2241, is 460 MPa to 710 MPa, and a tensilestrength at the room temperature, measured in accordance with JIS Z2241, is 560 MPa to 810 MPa.
 2. The nickel-containing steel according toclaim 1 comprising, as the chemical composition, by mass %: Ni: 11.5% ormore, and Mn: 0.50% or less.
 3. The nickel-containing steel according toclaim 1 comprising, as the chemical composition, by mass %: Ni: 11.5% ormore, wherein the average grain size of the prior austenite grains,measured in accordance with JIS G 0551, is 9.0 μm or less.
 4. Thenickel-containing steel according to claim 1, wherein an averageeffective grain size measured in the section of the thickness middleplane parallel to the rolling direction and the thickness direction is2.0 μm to 8.0 μm, wherein the average effective grain size is measuredby taking a sample from the steel after tempering, and observing five ormore visual fields at a magnification of 2,000-fold using an electronbackscatter diffraction analyzer, wherein an effective grain is definedas a grain surrounded by a grain boundary, and a grain boundary isdefined as a boundary of a metallographic structure having anorientation difference of 15° or more, and wherein a circle equivalentgrain size is obtained from multiple areas of effective grains by imageprocessing, and an average value of an obtained circle equivalent grainsizes represents the average effective grain size.
 5. Thenickel-containing steel according to claim 1, wherein an averageeffective grain size measured in the section of the thickness middleplane parallel to the rolling direction and the thickness direction is2.0 μm to 5.0 μm, wherein the average effective grain size is measuredby taking a sample from the steel after tempering, and observing five ormore visual fields at a magnification of 2,000-fold using an electronbackscatter diffraction analyzer, wherein an effective grain is definedas a grain surrounded by a grain boundary, and a grain boundary isdefined as a boundary of a metallographic structure having anorientation difference of 15° or more, and wherein a circle equivalentgrain size is obtained from multiple areas of effective grains by imageprocessing, and an average value of an obtained circle equivalent grainsizes represents the average effective grain size.
 6. Thenickel-containing steel according to claim 2 comprising, as the chemicalcomposition, by mass %: Ni: 11.5% or more, wherein the average grainsize of the prior austenite grains, measured in accordance with JIS G0551, is 9.0 μm or less.
 7. The nickel-containing steel according toclaim 2, wherein an average effective grain size measured in the sectionof the thickness middle plane parallel to the rolling direction and thethickness direction is 2.0 μm to 8.0 μm, wherein the average effectivegrain size is measured by taking a sample from the steel aftertempering, and observing five or more visual fields at a magnificationof 2,000-fold using an electron backscatter diffraction analyzer,wherein an effective grain is defined as a grain surrounded by a grainboundary, and a grain boundary is defined as a boundary of ametallographic structure having an orientation difference of 15° ormore, and wherein a circle equivalent grain size is obtained frommultiple areas of effective grains by image processing, and an averagevalue of an obtained circle equivalent grain sizes represents theaverage effective grain size.
 8. The nickel-containing steel accordingto claim 3, wherein an average effective grain size measured in thesection of the thickness middle plane parallel to the rolling directionand the thickness direction is 2.0 μm to 8.0 μm, wherein the averageeffective grain size is measured by taking a sample from the steel aftertempering, and observing five or more visual fields at a magnificationof 2,000-fold using an electron backscatter diffraction analyzer,wherein an effective grain is defined as a grain surrounded by a grainboundary, and a grain boundary is defined as a boundary of ametallographic structure having an orientation difference of 15° ormore, and wherein a circle equivalent grain size is obtained frommultiple areas of effective grains by image processing, and an averagevalue of an obtained circle equivalent grain sizes represents theaverage effective grain size.
 9. The nickel-containing steel accordingto claim 6, wherein an average effective grain size measured in thesection of the thickness middle plane parallel to the rolling directionand the thickness direction is 2.0 μm to 8.0 μm, wherein the averageeffective grain size is measured by taking a sample from the steel aftertempering, and observing five or more visual fields at a magnificationof 2,000-fold using an electron backscatter diffraction analyzer,wherein an effective grain is defined as a grain surrounded by a grainboundary, and a grain boundary is defined as a boundary of ametallographic structure having an orientation difference of 15° ormore, and wherein a circle equivalent grain size is obtained frommultiple areas of effective grains by image processing, and an averagevalue of an obtained circle equivalent grain sizes represents theaverage effective grain size.
 10. The nickel-containing steel accordingto claim 2, wherein an average effective grain size measured in thesection of the thickness middle plane parallel to the rolling directionand the thickness direction is 2.0 μm to 5.0 μm, wherein the averageeffective grain size is measured by taking a sample from the steel aftertempering, and observing five or more visual fields at a magnificationof 2,000-fold using an electron backscatter diffraction analyzer,wherein an effective grain is defined as a grain surrounded by a grainboundary, and a grain boundary is defined as a boundary of ametallographic structure having an orientation difference of 15° ormore, and wherein a circle equivalent grain size is obtained frommultiple areas of effective grains by image processing, and an averagevalue of an obtained circle equivalent grain sizes represents theaverage effective grain size.
 11. The nickel-containing steel accordingto claim 3, wherein an average effective grain size measured in thesection of the thickness middle plane parallel to the rolling directionand the thickness direction is 2.0 μm to 5.0 μm, and wherein the averageeffective grain size is measured by taking a sample from the steel aftertempering, and observing five or more visual fields at a magnificationof 2,000-fold using an electron backscatter diffraction analyzer,wherein an effective grain is defined as a grain surrounded by a grainboundary, and a grain boundary is defined as a boundary of ametallographic structure having an orientation difference of 15° ormore, and wherein a circle equivalent grain size is obtained frommultiple areas of effective grains by image processing, and an averagevalue of an obtained circle equivalent grain sizes represents theaverage effective grain size.
 12. The nickel-containing steel accordingto claim 6, wherein an average effective grain size measured in thesection of the thickness middle plane parallel to the rolling directionand the thickness direction is 2.0 μm to 5.0 μm, wherein the averageeffective grain size is measured by taking a sample from the steel aftertempering, and observing five or more visual fields at a magnificationof 2,000-fold using an electron backscatter diffraction analyzer,wherein an effective grain is defined as a grain surrounded by a grainboundary, and a grain boundary is defined as a boundary of ametallographic structure having an orientation difference of 15° ormore, and wherein a circle equivalent grain size is obtained frommultiple areas of effective grains by image processing, and an averagevalue of an obtained circle equivalent grain sizes represents theaverage effective grain size.